In a coupled systems complex with many physical processing nodes, for example, each comprising International Business Machines' System/390 parallel transaction server, the cost of input/output (I/O) connectivity between the different processors and their shared I/O devices is a significant component of the total computer system cost. Today, each processor has its own dedicated I/O subsystem structure, including its own channel subsystem with at least one channel connecting the processor to one or more of the shared I/O devices. Configuring enough channels, control unit adapters and (for IBM System/390-like systems) switches for full connectivity, results in using a much larger number of channels, control unit adapters, and switches than otherwise needed for performance and availability. A separate channel is required for each processing node, and switching (or multiple adapters at the control unit) is required to enable each processing node to communicate with a control unit connected to a desired input/output device.
In a conventional data processing system architecture, such as International Business Machines' Enterprise Systems Architecture/390, information is passed between main storage and input/output devices via a processor's dedicated channel subsystem. The channel subsystem uses one or more channel paths, each including a channel and one or more control units and serial links, as the communication links between its main storage and the input/output devices. The writing of data from main storage to input/output devices or the reading of data from input/output devices to main storage is accomplished by executing input/output operations. The execution of input/output operations is accomplished by the decoding and executing of channel command words by the channel subsystem and input/output devices.
As used herein, the phrase "physical processing node" comprises a processor having its own main storage or memory which is separate from and not accessible by other physical processing nodes in a computer system comprising a plurality of physical processing nodes. Conventionally, each physical processing node has its own channel subsystem and I/O processor(s) coupling its main storage to the shared I/O devices of the coupled systems complex.
There exists a need in the art for a computer system architecture and data processing approach which allow multiple physical processing nodes to obtain full connectivity to I/O devices using a smaller number of channels, control unit adapters, and switches, while still meeting performance and availability objectives.